Known fluid-flow machines which have a high-pressure and a low-pressure-steam region may be of single-cylinder or two-cylinder construction. Such fluid-flow machines, in particular steam turbines, are shown in 1997P03012 DE. The two-cylinder design does not belong to the technical field of the present invention and is therefore not described in more detail. The single-cylinder design consists of a rotor having two single-flow blade regions which point toward the respective casing ends. One blade region is designed as a high-pressure-steam blade region and another blade region is designed as a low-pressure-steam region. Inflowing live steam flows in the axial direction first of all through the blade region of the high-pressure-steam blade region. From there, the steam, which is now partly expanded, passes via a line to the intermediate-pressure-steam blade region.
In the high-pressure and intermediate-pressure regions, the specific volume, at a constant mass flow, increases relatively slightly in the course of the expansion. Starting from the transition region between intermediate pressure and low pressure (about 2 to 3 bar), the specific steam volume increases sharply, and the volumetric flow and thus the requisite flow area likewise increase sharply. Physical limits (e.g. strength) are encountered when realizing the flow area and this involves considerable construction outlay.
A disadvantage with these known embodiments having a high-pressure expansion region is that superheated steam comes in contact with the interior of a turbine end. To reduce the amount of steam escaping from the turbine, a plurality of sealing shells are arranged between outer casing and rotor. The high-energy steam between the sealing shells is partly fed back into blading regions of lower temperature for the thermodynamic optimization of the process. In this case, the introduction of the sealing shell steam into the blading regions leads to asymmetrical casing heating at the casing circumference, and this asymmetrical casing heating results in thermal stresses and deformations, i.e. distortion of the casing, which may possibly lead to grazing of blades on the casing.